The present invention relates to an image reading apparatus of the digital processing type and that can be generally applied to photocopiers and facsimile machines and the like, and more particularly relates to an image read apparatus provided with a function for maintaining the background tone of an image read by a plural number of line image sensors, as a white level.
Image reading apparatus that reads an image on one line using a plural number of image sensors have been proposed The configuration of a scanning system of a conventional image read apparatus of this type is indicated in FIG. 1(A), when seen from the direction of the sub scan.
Three CCD image sensors 11a, 11b and 11c are disposed at the lower portion of an original document 10. To the rear of the original 10 is disposed a document holder plate 12 to block out external light. A document holder surface of the document holder plate 12 is a color (such as white, yellow or the like) that generally has a high reflectivity so that is not read as an image. Light from a light source (not indicated in the figure) is reflected from the original document 10 and the document holder plate 12, and this reflected light from the original document 10 and the document holder plate 12 is irradiated via the lens system 13 to each of the CCD image sensors 11a, 11b and 11c. The output image signals from these three CCD image sensors 11a, 11b and 11c are serially synthesized to obtain the image signal for a one line.
In this type of image read apparatus, in order to always have the background portion of the original document reproduced as white, there is provided a function (called an AE function) to always maintain the background tone level of the image read by the image sensor, as a white level. This is to say that when the image sensor scans the original document, the peak level (a whiter level) of the image signal output from the image sensor is always held and the output signal from the image sensor is converted to image data (black and white data or tone data) using this held peak level as the white reference.
The output signal from the image sensor is for example, converted into image data in the following manner.
The whiter level signal that is held as the peak is input to the conversion reference input terminal (V.sub.REF) of an analog-to-digital converter (hereinafter termed an A/D converter) and the image signal from the image sensor is input to the analog input terminal of this A/D converter. The A/D converter converts the image signal between the black level (O.sub.V) and the white level (Reference input) into for example 6-bit (64 gradation) or 8-bit (256-gradation) digital image data. The reference input level of the A/D converter is always at the maximum value (3FH in the case of 6-bit) of the conversion data. In addition, even if there is a change in the background level of the original that is read by the image sensor, then the peak hold circuit follows that change. Accordingly, the image background tone level read by the line image sensor is maintained at a constant white level.
The 6-bit or the 8-bit image data converted by the A/D converter has the necessary image processing performed and is output from this image read apparatus.
Image read apparatus used in digital photocopiers are able to have many and varied types of original documents as the object. The size of the original documents that are handled can vary from large to small, as for example, original documents ranging from a maximum size of A0 to a minimum size of B5 can be read. In such a conventional image read apparatus, there is the following problem when small size original documents read.
As indicated in FIG. 1A, when a small-size original document 10 is placed in the center of a scan line, the peak level obtained from a central image sensor 11b differs as indicated in FIG. 1B for example, from the peak level obtained from the image sensors 11a and 11c on either side. Accordingly, the control so that the peak level in all of the image signals obtained from the three CCD image sensors 11a, 11b and 11c becomes the background level, is such that the white level of the document holder plate 12 becomes the background level if the background level of the original document 10 is low (such as in the case of diazocopy, original documents with colored backgrounds, secondary diazo originals and the like). This is to say that the background portion of the original document 10 is reproduced as an image having a constant tone.
In addition, when there is independent control by the AE function for each of the three CCD image sensors 11a, 11b and 11c, then the image signal from the central image sensor 11b is converted into image data the background level of the original document 10 as the reference. However, the image signals from the image sensors 11a and 11c on either side are converted into image data using the white level of the document holder plate 12 as the white level. In this case, the portions on both sides of the original document 10 and that is scanned by the image sensors 11a and 11c is reproduced as an image having a constant tone.
For example, in a image read apparatus that can read sizes up to A0, when an original of A3 size is read, the central image sensor 11b can only read 3l7.5 mm (including the overlap portion with the left and right image sensors 11a and 11c) of the 420 mm lateral width of the original document (where there is a total of 5000 picture elements and a density of 400 DPI). Accordingly, the 50-55 mm for the portions on either side of the original document 10 which is scanned by the image sensors 11a and 11c is reproduced as an image having a constant tone. In addition, the portion of the borders of the scanning area of the image sensors 11a and 11c with the scanning area of the image sensor 11b is expressed as different image data for the same areas. In this case, a missing of information occurs when such image data is binarized. When the image data appears as mid-tones, the tone of the border portions is different from that of the adjacent portions thereof.